Location and identification of faults in signaling transmission systems



H. H. BENNING Erm. 2,260,160

LOCATION AND IDENTIFICTIONl OF FAULTS IN SIGNALING TRANSMISSION SYSTEMS Filed April ze, 1940 Oct. 21, 1941.

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Patented` Oct. 21, 1941 LOCATION FAULTS 1N SYSTEMS Harvey H. Benning,

Frank A.

New York, N.

AND IDENTIFICATION oF slGNAmNG TRANSMISSION Lincoln Park, N. J., and

I eibe, Qnakertown, to Bell Telephone Laboratories,

Y., a corporation of New York Pa., assignors Incorporated, l

Application April 2s, 1940, serial No. 331,736

6 Claims.

This invention relates to fault location in a broad band wave transmission system, and more particularly to a method of and apparatus for both locating and identifying faults in such system.

Broad band wave transmission systems embody attended and unattended repeaters in each of two one-Way paths such that eachA path includes in tandem at least several unattended .repeaters positioned between two attended repeaters. Usually, the unattended repeaters are serviced from an attended repeater so that in case of the occurrence of a fault on the system it may be cleared by an attendant going out from the latter. Such system is so arranged that wave transmission is effected on individual coaxial conductors simultaneously in opposite directions. Each repeater in each one-way path provides for the transmission of a plurality of signals depending on the number of carrier channels embodied in the system. Among these signals is a pilot signal which is utilized to indicate at an attended repeater the failure of an unattended repeater in either direction of transmission. Heretoiore, this has been accomplished by associating all unattended repeaters with a direct current circuit such that upon the failure of an unattended repeater a short circuit would be ap plied across the direct current circuit at that point.

This would actuate a signal at an attended'repeater to announce to the attendant thereat the failure of one of the unattended repeaters. Promptly, the attendant would effect .loop-resistance measurements on the direct current path to locate the unattended repeater involved in the failure. A single indication at the attended repeater Would mean that the failure could be on either the eastward or the westward path and, of course, the attendant would have no way of knowing the fact. A signal individual to each path and common to all unattended repeaters in tandem in the respective eastward and westward paths may be used to indicate failure in a certain path. Thus, two indicators would be required to isolate the individual unattended repeater which had failed.

In either case, siderable time at the unattended repeater involved in the failure in testing to ascertain the the attendant may consume connature of the fault before the proper steps may be taken to effect clearance thereof and restore normal service on the system. Prompt action on the part of the attendant is imperative in order to maintain reliable and eiilcient transmission and, in addition, to reduce to a minimum the outof-service periods of the regular equipment. Accordingly, this invention contemplates an arrangement for apprising the attendant not only the location of a fault but also the nature thereof, preparatory to his going out to the attended repeater involved in the fault to effect clearance thereof.

The main object of the invention is to provide a broad band transmission system with a facile arrangement for both locating and identifying certain faults that may occur therein.

Another object is to accomplish expeditiously the fault location and identification.

A further object is to reduce to a minimum the time required to clear the faults.

Heretofore, the failure of an unattended rel peater in a broad band transmission system ern-- bcdying in tandem a plurality of attended and unattended repeaters individual to one-way westward and eastward paths has been announced to an attendant located at an attended repeater by an arrangement comprising the association of all unattended repeaters with a normally open direct current circuit such that the failure of one of the unattended repeaters would apply a short cil'- cuit across the direct current circuit to actuate an alarm at the attended repeater. By means of loop-resistance measurements on the Vdirect current circuit the attendant would be able to i'solate the faulty unattended repeater.

In accordance with this invention, a pair of sensitive relays is associated with the direct current circuit and both one-way -ampliers at each unattended repeater so that one contact of each sensitive relay may serve to apply a closure circuit to normally open direct current circuit, depending on whether the pilot current is too high or too low. A resistance combination comprising four different amounts is associated with the sensitive relays such that for each of four diiferent conditions of the pilot current on both one-way paths at each unattended repeater one amount of resistance is connected in the closure circuit in series with the closed contact of rent sources 28 and 31 and identify a fault involving acer-tain strength of/f" the pilot current in a certain one-way path at l each unattended repeater.-

The invention will be readily understood from the following description taken together with the accompanying drawing in which:

Fig. 1 is a box representation of the invention shown in connection with a broadband wave transmission system, and

Fig. 2 is a schematic circuit illustrating the lnvention embodied in each unattended repeater of the system shown in Fig. 1.

Referring to' Fig. 1, one attended repeater station I8 is connected with remote attended repeater station II by a, pair of coaxial conductor lines I2 and I3. Intermediate both attended repeater stations are positioned a plurality of unattended repeater stations R1, Rz, Ra and R4, each of which includes a WE one-way amplifier I4 and an EW one-way amplifier I5. The attended repeater stations may be either terminating or intermediate stations. Such equipped coaxial peater at which the closure circuit occurred but also to ascertain (l) whether it happened either on the EW or WE coaxial conductor line and pilot current which is utilized in this illustration, whether such current is too high or too low on the respective lines. y Referringto each unattended repeater of Fig. 1, it will be observed that equal resistances 39, 39 are inserted in series in the circuit I6 which also has connected thereto and to the EW and WE ampliliers certain switching apparatus 44. Viewed from both the primary and secondary control repeaters, each unattended repeater includes the pair of resistances 39, 39 and the apparatus 44.

Referring to Fig. 2, each apparatus 44 comprises connected to the output of the WE ampliner I4 transformers 23 and 24 which are tuned by capacities and 28 to select a pilot current for application to the input vertices of a full wave rectiiier bridge 21 comprising rectiers 28 (2) in the case of and 29 forming two arms and capacities 38 and conductor lines may be operated for broad band i wave transmission in the manner pointed out in the patent of L. Espenschied et al. No. 1,835,-

v031 granted December 8, 1931. A direct current circuit I6 is provided between attended repeaters I8 and II both of which will be hereinafter identified as the respective primary and secondary control repeaters.

At the primary control repeater I8, the circuit I6 is normally terminated in an alarm I 'I through a manual switch I8 which may be actuated to connect the circuit I6 to testing equipment I 9. Associated with the latter and disposable in series in the circuit I6 are a direct current source 28 and an adjustable resistance 2l which serves to vary the amount of direct current flowing in the circuit I6 for a purpose that will be later explained with respect to loop-resistance measurements made from the primary control repeater I8.

` At the secondary control repeater II, the cir- L cuit I6 is normally terminated in an open circuit by a switch 22 which may be actuated to connect thereto testing equipment 36. Associated with the latter and disposed in series there- 1 with are a direct current source 31 and an adjustable resistance 38 which serves to vary the amount of direct current flowing in the circuit I6 for a purpose that will be subsequently del scribed from the standpoint g measurements made from 1 repeater II.

the secondary control I 9 andk 36 are identirespective direct curadiustable resistances 2I and 38'are utilized, in response to actuations The testing equipments cal and together with the 1 `o1' the alarm I 1 due to a closure of the circuit I6 g at each unattended repeater in a manner that gwill be subsequently explained. to make on the ,circuit I6 loop-resistance lmeasurements which will indicate the location of the closure circuit. ,An arrangement embodying the above apparatus for making such resistance measurements is disclosed in the patents to E. I. Green et al., Nos. ,2,018,850 and 2,018,851 both of which were `granted October 29, 1935. In accordance with theinvention, it-will be possible to locate not only the unattended reyof loop-resistancev may be associated erably of 'the copper-oxide type are oppositely poled so as to effect rectication of each half of the selected pilot current voltage. The output of the EW amplifier I5 is applied through identical transformers and rectifier lbridge to operating coil 34 of a sensitive relay 35.

Sensitive relay 33 embodies an armature 48 adapted to be moved by the operating coil 32 into engagement with the respective upper and lower contact members 4I and 42, and a restoring coil 43. Sensitive relay 35 comprises an armature adapted tovbe actuated by the operating coil 34 into engagement with the respective upper and lower contact members 46 and 41, and a restoring coil 48. Both sensitive relays are identical instructure and preferably of the type disclosed in the patent of J. W. Beyer et al., No. 2,066,514, issued January 5, 1937.

The respective restoring coils 43 and 48 may be independently connected in series with a source 58 of direct current voltage by means of a righthand contact 5I and an inner left-hand contact 52 of an electromagnetic holding relay 53. The upper contact members 4I and 46 are joined by -a lead 55 and the'lower contact members 42 and 41 are connectedv by a lead 56, both of which leads are bridged by a lead 51 embodying a 100-y ohm resistance 6I. The armatures 48 and 45 are connected by a lead 58 embodying a 20o-ohml resistance 68. The function of the resistances 68 and 6| will be presently explained.

Terminal lead of the the relay 53 may be connected to the lead 51 by means of the outer left-hand contact 62 and is joined to the lead 58 connecting the armatures 48 and 45 of the sensitive relays 33 and 35. Terminal lead 66 of the operating winding of the relay 53 is applied to one conductor of the circuit I6 whose other conductor is applied by a lead 68 to the lead 56.

In addition to the fact that the loop resistance o1' the circuit I6 measured from either the primary or secondary control repeater to each unattended repeater is diierent for each of the latter so as to enable immediate identification thereof as pointed out in both patents of E. I. Green et al., supra, the resistances 68 and 6I with the respective contact members of the sensitive relays 33 and 35 in the closure circuit applied across the circuit I6 at operating winding of I l 2,260,160 vthe unattended repeaters in a manner that will be presently described so as to enable differentiation between the following conditions of the pilot current thereat: f

Table A.

Ohms resistance in series .with contact members oi sensitive rela s 33 an 35 Pilot current fault the location of the application of the closure circuit and the cause thereof, as such chart includes above Table A and corrections for temperature variations.

The operation of Figs. 1 and 2 is as follows:

If, for example, the strength of the pilot current on EW coaxial conductor I3 is increased 3 decibels or more above normal, the level of the rectified current supplied to the operating coil 34 of the sensitive relay 35 is increased sul` ciently to effect engagement between the. armature 45 and lower contact member 41.v This connects to the circuit i8 a closure circuitcomprising leads 68 and 5B, contact member 41 and armature 45 in engagement therewith, lead 58, terminal lead 65 and operating winding of relay 53 and terminal lead BB of relay 53. This operates the alarm I1 at the primary control repeater I to announce to an attendant thereat the occurrence of a fault at one of the unattended repeaters.

The attendant may now readily determine by loop-resistance measurements (a) the unattended repeater at which the closure circuit was applied to the circuit i5, (b) whether the closure circuit was due to a faultl on the EW' or WE co- 4 axial conductor line, and (c) whether the strength of the pilot current vwas too high or too low relative to normal. This may be accomplished as follows:

First step, the attendant at the primary control repeater I0 measures the loop resistance of the circuit i6 including the closure circuit placed thereacross and identified above. Such measurement will include 1. Resistance of the circuit i5 including the series resistances 39, 39 and extending between the primary control repeater l0 and the unattended repeater at which the closure circuit occurred. l

2. Resistance of the operating winding of relay 53 and of the leads extending therefrom to the circuit i6 and to the contact member of the sensitive relay whose armature was actuated into engagement therewith.-

3. Resistance 60 or 6i, individual or in combination, or none, connected in series with the contact member of the sensitive relay whose operated armature is in engagement therewith;

Immediately upon the operation of the alarm to announce the occurrence of the fault but before proceeding with the loop-resistance measurement mentioned above, the attendant at the primary control repeater i0 notifies the attendant at the secondary control repeater Il so that a loop-resistance measurement may also be made from the latter repeater. Such measurement may be achieved by actuating the switch 2! at the secondary control repeater il to connect the testing equipment 35' to the circuit i5. Hereinafter it is to be understood that the description of the operation of Figs. l and 2 is equally pertinent to loop-resistance measurements made independentlyi'rom either the primary cntrol repeater I0 or the secondary control repeater Ii, or simultaneously from both thereof.

Second step, the measuring current flowing in ,the circuit I6 is gradually increased to a predetermined value which serves only to operate the relay 53 locatedr in the nearest unattended repeater at which the closure circuit was applied to the circuit i5; This is accomplished because the resistances 39, 38 embodied in the circuit I8 vare proportioned to effect such operation. The operation of relay 53 closes its contacts 5I, 52 and 62. The closing of contacts 5i and 52 applies battery '50 to the respective re- `storing coils 43 and 48 of the sensitive relays 33 and 35 which coils are energized to return the operated armature 45 of the sensitive relay 35 2 `to normal, as it was the only operated armature and therefore the only one capable of being returned to normal. 'I'his restoring operation serves to interrupt the closure circuit including the resistances 60 and BI, individual or in combination, or none, connected in series with the respective contact members 4|, 42, 45 and 41 of the sensitive relays 33 and 35, and to substitute therefor a short circuit which will be presently described.

Third step, the attendants at the respective primary and secondary control repeaters I0 and II again measure the circuit I5. Now, such measurement would include the portion of the circuit i6 extending between the respective primary and secondary control repeaters I0 and Il and the unattended repeater at which the closure circuit occurred together with the short circuit applied theretact 62 of operated relay 53, operating winding of relay 53 and terminal lead 65 of the latter.

Fourth step, both attendants note any difference between the loop-resistance measurements made in the first and third steps and further that such difference would be equivalent tothe value of the resistance or 6I, individual or in combination, or none, connected in the closure circuit in series with one of the contact members 4I, 42, 45 and 41 of the sensitive relays 33 and I5 as mentioned above under the tlrst step. Thereafter, reference to resistance-distance will enable both attendants to determine (a) the unattended repeater at which the closure circuit was applied to the circuit I5. (b) whether the application of the closure circuit was due to a pilot current variation on either the EW or the WE coaxial conductor line, and (c) whether the strength of the pilot current was too high or too low. As there was -ohms resistance in the closure circuit inseries with the contact member 41 of the sensitive relay 35, reference to Table A above will show that this indicates that the strength of the pilot current on the EW coaxial conductor line was too high.

Fifth step. the measuring current of the circuit i6 is reduced to a certain value which is insufiicient to maintain the relay 53 in the operated condition and therefore this relay 'is returned to the unoperated condition. If the application of the closure circuit tothe circuit I5 the loop resistance of l across comprising leads 68,56 and 51, closed conattendant with the proper information at his disposal would proceed to the faulty unattended repeater to remove in a minimum time the cause giving rise to the application of the closure circuit to the circuit I6.

If the strength of the pilot current on EW coaxial conductor line I3 is decreased by 3 decibels or more below normal, the level of the rectiiied current supplied to the operating coil 34 of therelay 35 is decreased suil'iciently to effect engagement between the armature 45 and the contact member 46. This connects to the circuit I6 a closure circuit comprising leads 68 and 56, lead 51 including 10D-ohm resistance 6|, lead 55, contact member 46 and armature 45 in engagement therewith, lead 58, terminal lead 65, operating winding of relay 53 and terminal lead 66. This operates the alarm I'I at the primary control repeater I0 to announce to the attendant thereat the occurrence of a fault at one of the unattended repeaters. Both attendants repeat the above loopf resistance measurements which, in this case, in-

dicate by the diierence between the results of the rst and third steps that the 1D0-ohm resistance 6| was connected in the closure circuit' in series with the contact member 46. Reference to the resistance-distance chart will show that this indicates that the pilot current on the EW coaxial conductor liney I3 was too low.

If the strength of the pilot current on WE coaxial conductor line I2 is increased 3 decibels or more above normal, the level of the rectified current supplied to the operating coil 32 of the relay.

33 is increased suilicientlyA to effect engagement between the' armature 40 and contact member 42. .l

'I'his connects to the circuit I6 a closure circuit comprising leads 68 and 56, contact member 42 and armature' 40 in engagement therewith, lead 58 including 20G-ohm resistance 60, terminal lead 65, operating winding of relay 53 and terminal lead 66. This operates the alarm I1 at the primary control repeater I to announce to the attendant thereat the occurrence of -a fault at one of the unattended repeaters. Both attendants re- 1 peat the above loop-resistance lmeasurements which, in this case, indicate by the difference between the results of the first and third steps that the 20G-ohm resistance 60 was connected in the closure circuit in series with the contact member 42. Reference to the resistance-distance chart will show that this indicates that the pilot current on the WE coaxial conductor line I 2 was too l high.

If the strength of the pilot current on WE coaxial conductor line I2 is decreased 3 decibels or more below normal, the level of the rectified curi rent supplied to the operating coil 32 of the relay 33 is decreased suciently to effect engagement 5 between the armature 40 and contact member 4I. This connects to the circuit I6 a closure circuit comprising leads 68 and 56, lead 51 including the i 10D-ohm resistance 6I, lead 55, contact memberv 34| and armature 40 in engagement therewith,

lead 58 including the 20G-ohm resistance 60, terat unattendedv repeaters one of the unattended repeaters. Both attendants repeat the above loop-resistance measurements which, in this case, indicate by the diil'erence between the results of the first and third steps that the 10U-ohm resistance 6I and the 200- ohm resistance 60 were. both connected in the closure circuit in series with contact member 4I. Reference to the resistance-distance chart will show that this indicates that the pilot current on the WE coaxial conductor line I2 was too low.

In the above it was assumed for illustration that a fault occurred at only one oi.' the four unattended repeaters at one time so that the loopresistance measurements computed independently from the respective primary and secondary control repeaters III and cate readily the same unattended repeater involved in the fault. The occurrence of similar faults simultaneously at unattended repeaters R1 and R4, or any combination of two unattended repeaters, would be located and identified in accordance with the procedure described above, except the attendant at primary control repeater III would measure the loop resistance only with respect to unattended repeater R1 while'the'attendant at the secondary control repeater II would measure the loop resistance only in connection with unattended repeater R4. This is so because increasing the current in the circuit I6 as pointed out in the second step would effect the operation only of the holding relay 53 embodied in the unattended repeater at which the closure circuit was eiected and which is nearest to primary control repeater III orthe secondary control repeater II. l

Accordingly, primary control repeater I6 would cause the operation of the holding relay 53 at unattended repeater R1 while secondary control repeater II would cause the operation of the holding relay 53 at unattended repeater R4. Hence, the loop-resistance measurements accomplished by the primary control repeater I6 would relate exclusively to unattended repeater R1 and those performed at secondary control repeater I I would relate exclusively tounattended repeater ,Re Reference to the resistance-distance chart would readily locate the unattended repeaters involved in the faults and identify the respective causes thereof.

'I'he simultaneous occurrence of the same fault lRi, R2 and R4, or any combination of these or more unattended repeaters, would be located and identified in accordance with a combination of the routines mentioned previously in connection with faults involving (a) a single unattended repeater, and (b) two unattended repeaters. Thus, primary control repeater III would take care of the loop-resistance measurements relating to unattended repeater R1; secondary control repeater I I would handle the loopresistance measurements concerning unattended repeater R4; and either primary control repeater I0 or secondary control repeater I I would dispose of the loop-resistance measurementsv involving unattended repeater R2, depending on which of the former was first available for this purpose. Reference to the resistance-distance chart vwould readily locate the unattended repeaters involved in the faults and identify the respective reasons therefor.

II would both serve to lo-l Table B Ohms resistance in series Pilot current fault sensitive relays 33 and 35 EW high-WE high 'Ow and will EW low-WE low 100cv and 300 EW li-WE low u and 300. EW low-WE high 100 and 203 In each of the above cases, a loop-resistance measurement would show the condition of lowest resistance which measurement would serve to direct an attendant to the unattended repeater involved in the fault, in the manner indicated above, but only with the infomation that thereat a single fault existed. At the proper unattended repeater a routine check of both sensitive relays 33 and 35 would show the existence of the second fault. Thereupon, appropriate adjustments could be made to eifect a clearance of both faults on the occasion of one trip on the part of the attendant.

What is claimed is:

1. In a signaling system comprising in combination two one-way paths, at leasttwo attended repeaters and a plurality of unattended repeaters therebetween disposed in tandem in said paths, means at each repeater to transmit a plurality of signaling currents including a pilot current on each path, circuit means extending from each attended repeater to each unattended repeater, indicating means connected to said circuit means at an attended repeater, means at each unattended repeater responsive to certain changes of the condition of the pilot current on each oneway path to apply a closure circuit to said circuit means whereby said indicating means is actuated to announce the occurrence oi said changes, and means to make resistance measurements on said circuit means tu ascertain the location of said closure circuit; means included in said closure circuit to identify said certain changes of the condition of the pilot current and the oneway path involved in said closure circuit, said identifying means comprising a plurality of different amounts of resistance each of which amounts represents an increase or decrease in the strength of the pilot current on each oneway path, means responsive to a predetermined increase` or decrease in the strength of the pilot current to dispose one or more said amounts of resistance in said closure circuit to make one resistance measurement and further means responsive to a change in the condition of said circuit means and said closure circuit to remove one or more said amounts of resistance from said closure circuit to make a second resistance measurement, the difference between both resistance measurements being equivalent to one or more said amounts of resistance representing a certain change of the condition of the pilot current on a certain one-way path.

2. In a signaling system comprising in combination two one-way paths, at least two attended repeaters and a plurality of unattended repeaters therebetween disposed in tandem in said paths. means at each repeater to transmit a plurality ci signaling currents including a pilot current on each path, circuit means extending from each measurements being attendedrepeater to each unattended repeater, indicating means connected to said circuit means at an attended repeater, means at each unattended repeater responsive to certain changes ci the condition of the pilot current on each oneway path to apply a closure circuit to said circuit means whereby said indicating means is actuated to announce the occurrence of said changes, and means to make resistance measurements on said circuit means to ascertain the location of said closure circuit; means included in said closure circuit to identify said certain changes of the condition of the pilot current and the one-way path involved in said closure circuit, said identifying means comprising a plurality of resistance elements disposable singly or in combination in said closure circuit to represent a too high or too low condition of the pilot current on a certain one-way path, and electromagnetic means responsive to a change in the condition of said circuit means and said closure circuit to interrupt said closure circuit and to substitute therefor a short circuit, the diilerence between resistance measurements on said circuit means involving the conditions of the closure circuit and of the short circuit being equivalent to the resistance elements singly or in combination representing a certain change of the condition of the pilot current on a certain one-way path.

3. In a signaling system comprising in combination 4two one-way paths, at least two attended repeaters and a plurality of unattended repeaters therebetween disposed in tandem in said paths, means at each repeater to transmit a plurality of signaling currents including a pilot current on each path, circuit means extending from each attended repeater to each unattended repeater, indicating means connected to said .circuit means at an attended repeater, means at each unattended repeater responsive to certain conditionsof the pilot current to apply a closure circuit to said circuit means whereby said indicating means is actuated to announce the occurrence of said conditions, and means to make resistance measurements on said circuit means to ascertain the location of said closure circuit; means included in said closure circuit to identify said certain conditions of the pilot current and the one-way path involved in said closure circuit. said identifying means comprising a plurality of diiferent amounts of resistance each of which amounts represents a certain condition of the pilot current on a certain one-way path, electromagnetic means responsive to a certain condition o! the pilot current in each one-way path to connect one or more amounts of resistance in the closure circuit to make one resistance'measurement and further electromagnetic means responsive to a change in the condition of the closure circuit to remove the one or more amounts of resistance therefrom to make a second resistance measurement, the difference between the two resistance equivalent to an amount of resistance representing a certain condition of the pilot current on a certain one-way path.

. 4. In a signaling system comprising in combination two one-way paths, at least two attended repeaters and a plurality of unattended repeaters therebetween disposed in tandem in said paths, means at eachrepeater to transmit a plurality of signaling currents including a pilot current on each path, circuit means extending from each attended repeater to each unattended repeater, indicating means connected to said circuit means at an attended repeater, means at mined variation of n sure circuit. and

.said paths, means at way path to apply vance measurements on said certain condition cuit. said identifying means each unattended repeater responsive to a certain condition of the pilot current on each one-way path to apply a closure circuit to said circuit means whereby said indicating means is actuated to announce the occurrence of said condition. and

volved in said closure circuit, said identifying means comprising a plurality of different amounts of resistance each of which amounts represents a certainl condition ofpilot current on a certain one-way path, -means responsive to a predeterrent on a certain one-way path to connect one the strength of the pilot curor more said amounts of resistance in said clomeans responsive to a change in the condition of said circuit means and the closure circuit applied thereacross to remove from Vsaid closure circuit said one or more amounts of resistance, the difference between resistance measurements including one or more said amounts of resistance and excluding one or more said amounts of resistance being equivalent to an amount of resistance representing a certain condition of the pilot current on a certain oneway path.

. 5. In a signaling system comprising in combination two one-way paths, at least two attended repeaters and a plurality of unattended repeaters therebetween disposed in tandem in each repeater to transmit a plurality'of signaling currents including a pilot ncurrent on each path, circuit means extending from each attended repeater to each unattended repeater, indicating meansconnectcd to said Vcircuit meansat an attended repeater, means at each unattended repeater responsive to a certain condition of the pilot current on each onea closure circuit to said circuit means whereby said indicating means'is actuated to announce the occurrence oi' said condition, and means connected to said circuit means at an attended repeater to make resistsaid circuit meansl to ascertain the location of said closure circuit, means included in the closure circuit to identify of the pilot current and the one-way path involved in said closure cir` comprising a plurality of different amounts of resistance each of which amounts represents a certain condition of the pilot current on a certain one-way path, electromagnetic means responsive to a predetermined variation in the strength of the pilot current on Aa certain one-way path to connect one or more said 4amounts of resistance in the closure circuit to make one resistance measurement, and electromagnetic means responsive to a change of condition on'said circuit bination two one-way paths, at least two attended repeaters and a plurality of unattended -repeaters therebetween disposed in tandem in said paths, means at each repeater to transmit a plurality of signaling currents including a pilot current on each path, circuit means extending from each'attended repeater to each unattendedv repeater, indicating 'means connected to said circuit means at an attended repeater, means'at each unattended repeater responsive to a certain condition of the pilot current on each one-way path to vapply a closure circuit to said circuit means whereby said indicating means is actuated to announce the occurrence of said conditions, and means to make resistance measurements on said circuitmeans to ascertain the location of said closure circuit; means associated vwith said closure circuit to identify said certain condition of the pilot current volved in said closure circuit, said identifying means comprising a plurality of different amounts of resistance to represent different conditions of the pilot current on both one-way paths, a pair of sensitive electromagnetic relays arranged so thatone thereof is connected to one one-way path andy the second thereof to the other-onetermined variation in the strength of the pilot current on said. one-way paths to connect one amount of resistance in the closure circuit, and a.- holding Velectromagnetic relay operated in response to a change in the condition of said circuit means and said closure circuit applied thereacross to return said operated sensitive relays to the unoperated condition thereby interrupting said closure circuit while at the same time applying a short circuit to said circuit means, the difference in resistance measurements involving said closure circuit and said short circuit being equivalent to an amount of resistance representing the condition of the pilot current and oneway path involved in the application of said closure circuit to said circuit means.

i HARVEY n. BENNING.

FRANK A. LEBE.

mean.d and closure circuit applied thereacross to interrupt said closure and the one-way path in- 

